1. Field of the Invention
The present invention relates to visible light sensitive imaging systems, and in particular to photothermographic imaging systems comprising a nitrate salt, a leuco dye, an initiator, a sensitizing dye, a binder, and an optional organic acid.
2. Information Disclosure Statement
Many processes and compositions use leuco dyes to provide optical densities in the imaged article. For example, U.S. Pat. No. 4,017,313 uses a combination of a photosensitive leuco dye, a photosensitizer for the dye, an aromatic aldehyde and a secondary or tertiary amine. Other photosensitive systems using leuco dyes are included in U.S. Pat. Nos. 3,390,997, 2,884,326, and 2,772,284. The mechanism of these last two patents are disclosed in "Aromatic Aldehyde-Leuco Dye Photooxidation" (Hartzler, H. D. Pure Appl. Chem. 1977, 49, pp 353-356.)
Light-Sensitive Systems (Kosar, J.; John Wiley and Sons: New York, 1965: p 369), describes print-out photosensitive systems comprising a binder, leuco dye, organic halogen releasing compound and a photosensitizing dye. Because these are printout systems, there is no thermal amplification.
A great many photosensitive materials have been used in different imaging processes utilizing various photoinitiated phenomena such as photohardening of polymerizable materials (e.g., negative-acting printing plates), photosolubilizing materials (e.g., positive-acting printing plates), light initiated diazonium salt coupling reactions (e.g., diazonium microfilm), etc. A class of iodonium photoinitiators for both cationic and epoxy polymerization (e.g., U.S. Pat. Nos. 4,026,705 and 3,981,897) has also been proposed as equivalent to other photoinitiators in certain ethylenically unsaturated printing plate compositions (e.g., U.S. Pat. No. 3,741,769).
Photothermographic imaging systems are well known in the art. By definition photothermographic systems are light-sensitive imaging systems which are thermally developed. Photothermographic systems typically require temperatures in the range of 80.degree. to 200.degree. C. A number of imaging systems employ photosensitive compounds, leuco dyes or bleachable dyes, and nitrate salts to generate color images. Imaging systems comprising a leuco dye or bleachable dye, nitrate ion, and diazonium salts in a binder are disclosed in U.S. Pat. No. 4,370,401. In those cases wherein a leuco dye system is employed, a photothermographic, negative-acting imaging system is provided; that is, the optical density in the final image is more dense in areas which are light struck than in areas which are not light struck. Conversely, in those cases wherein a bleachable dye system is employed, a photothermographic, positive acting imaging system is provided. That is, the optical density in the final image is more dense in areas which are not light struck than in areas which are light struck. The bleachable dye used in these cases does not serve in the role of a sensitizer.
Related imaging compositions comprising a diazonium salt and leuco dye in a binder are disclosed in U.S. Pat. No. 4,394,433. These unamplified compositions are positive-acting photothermographic compositions, and differ fundamentally from the compositions of the present invention, which are amplified by the action of a nitrate salt.
Additional light-sensitive, thermally developable imaging systems are known. U.S. Pat. No. 4,460,677 describes a thermally developable imaging system comprising a leuco dye, nitrate ion, and a spectrally sensitized organic compound having photolyzable halogen atoms. Similarly, U.S. Pat. No. 4,386,154 describes a thermally developable imaging system comprising a leuco dye, a nitrate salt, and a spectrally sensitized compound selected from (1) aromatic iodonium salts and (2) photolyzable organic halogen compounds. Both of these compositions act as negative image forming systems in that the greatest image density is formed upon heat development in the light struck areas. The latent images are formed upon exposure to visible light and visible images are formed by heat development. The color fidelity and contrast of both of these systems is reduced by the presence of sensitizer stain, that is color due to the sensitizer which remains in the system, and to colored by-products from reacted sensitizer in light exposed regions. This sensitizer stain aesthetically and functionally detracts from the image.
Light-sensitive, thermally developable imaging systems are also described in several Japanese patents.
Japanese Pat. No. 77,025,330 pertains to a two component positive acting imaging composition comprised of an oxazine or phenothiazine leuco dye (BLMB) mono- or disubstituted with a dialkylamino group, and an oxidizing agent such as nitrate ion. Said composition is not light sensitive in the visible region of the spectrum, and does not employ diaryliodonium salts or organic compounds having photolyzable halogen atoms.
Japanese Pat. No. 77,004,180 describes the use of triplet sensitizers for BLMB. Suitable sensitizers are aromatic carbonyl compounds and aromatic nitro compounds. That patent describes both negative and positive systems, and is a counterpart to Japanese Pat. No. 77,025,330. Said composition is not light sensitive in the visible region of the spectrum, and does not employ diaryliodonium salts or organic compounds having photolyzable halogen atoms. Further, triplet sensitizers are by definition not photobleached during energy transfer.
Japanese Pat. No. 76,035,847 describes photosensitive heat-fixable recording materials containing a free radical producing organic halogen compound, leuco dye and a base. This is a negative-acting system, and contains no oxidizer.
Japanese Pat. No. 77,025,088 describes photosensitive compositions containing an acid-sensitive leuco dye (e.g., naphthospiropyran), and a photochemical acid generating agent which is a mixture of an organic halide (e.g., CBr.sub.4), with a furan containing compound.
Japanese Pat. No. 79,001,453 describes a photothermographic material which contains an oxidizer, a compound which reacts with the oxidizer to change or develop color, and a compound which deactivates the color developer either in exposed or unexposed regions. Images can be either positive or negative, and do not employ sensitizers or diaryliodonium salts or organic compounds having photolyzable halogen atoms, which are components of the present invention, and which activate rather than deactivate color development. The light sensitive materials used were colorless or nearly colorless aryl quinones and ultraviolet light sources were used for imaging. Additionally, the light-sensitive materials used were not photobleachable. Furthermore, the compositions of the present invention show considerable amplification in both exposed and unexposed regions due to the presence of an added UV sensitive initiator (i.e., diaryliodonium salt or a organic compound with photolyzable halogen atoms), and therefore do not function by deactivation in the sense of Japanese Pat. No. 79,001,453.
Decolorizable imaging systems comprising a binder, nitrate salt, acid, and dyes are disclosed in U.S. Pat. Nos. 4,336,323 and 4,373,020. These systems are particularly useful as antihalation layers in photothermographic systems where the development temperature acts to bleach the dye.
The use of photobleachable dyes including o-nitroarylidene dyes as antihalation or acutance dyes is known in the art: U.S. Pat. Nos. 4,111,699; 4,271,263; 4,088,497; 4,033,948; 4,028,113; 3,988,156; 3,988,154; 3,984,248; 3,615,432 (RE28,225). The use of photobleachable dyes in this manner is unrelated to their function in the present invention. Additionally, it was found that o-nitroarylidene dyes are desensitizing to silver halide imaging systems.
Spectral sensitization of silver containing photothermographic compositions has been disclosed in U.S. Pat. No. 4,461,828; however, no mention is made of photobleachable sensitizers.
U.S. Pat. No. 4,713,312 teaches photobleachable sensitizers useful in the wavelength range of 390-500 nm for free radical polymerization to reduce background sensitizer stain in an imaging system based on photosensitive microcapsules. In the case of a full color imaging system, for which sensitivity to the entire visible spectrum of 400-700 nm is necessary, this patent does not provide for complete removal of residual sensitizer stain.